South Africa were left staring at yet another disappointment in a major tournament after Australia found an unlikely new bowling hero in Xavier Doherty.

The left-arm spinner from Tasmania, one of several options tried and discarded by the Aussies during England's Ashes triumph two winters ago, had not played in their previous three games here. But he was drafted in to provide an extra spin option on one of the increasingly worn Premadasa pitches and given the new ball – and he took a wicket in each of his first two overs.

Richard Levi, a chunky opener who has made little impression either here or in South Africa's t20 series in England, was bowled by his third ball before Doherty claimed the higher-calibre scalp of Jacques Kallis with a beauty which gripped and spun sharply to find the outside edge. The Proteas, who had lost their previous Super Eights fixture to Pakistan in the last over, were always chasing the game after that.

The celebrations that followed Hashim Amla's dismissal gloving an attempted hook at Shane Watson suggested it had been one of many cunning plans that have paid off for Australia in this competition. Doherty then returned to have JP Duminy stumped by a quicker ball fired between the left-hander's legs, and it was only an unorthodox unbeaten 32 off 19 balls from the former Derbyshire spinner Robin Peterson that set Australia a remotely challenging target.

They seemed to be wobbling at 10 for one in the fourth over when Morne Morkel bowled David Warner with a beauty. But Watson appears equal to any challenge in this tournament, having set himself a personal goal of emerging as its dominant player. He dominated a second-wicket stand of 99 with Mike Hussey, and his dismissal for 70 from 47 balls left the powerful Queenslander with 234 from 143 in his four innings, having hit 17 fours and 15 sixes – in addition to taking 10 wickets for 118.

There were rare fielding fumbles from South Africa as they faced likely elimination. Watson was dropped on 52 by Wayne Parnell running in from long off and perhaps the heavy load that AB de Villiers has been carrying as wicketkeeper and captain took its toll when he fluffed the chance to stump Hussey in the 17th over.

Hussey launched Peterson's next ball for six, and Cameron White sealed the win in the next over with 14 balls to spare. Rather like England in the Caribbean two years ago, these unheralded Aussies are starting to develop real momentum.